To provide a high tensile strength hot rolled steel sheet having a yield ratio of ≤60% in the temperature range of 150 to 300°C and excellent in warm pressing characteristics and to provide a producing method therefor.
A steel stock having a composition containing 0.01 to 0.12% C, ≤2.0% Si, 0.01 to 2.0% Mn, 0.001 to 0.1% Al and 0.0040 to 0.0200% N and moreover containing 0.001 to 0.1% Ti and ≤0.01% B is heated, is subjected to hot rolling so as to control final finish rolling temperature FDT to (Ar3+100°C)-(Ar3-10°C) with a draft of ≥15% at final stand, is cooled at a cooling rate of ≥40°C/s within 0.5 sec after the completion of the hot rolling and is coiled at a coiling temperature of 650 to 350°C. In this way, a high tensile strength hot rolled steel sheet having a structure composed of ferrite with the average crystal grain size of ≤8 μm as the main phase and also containing solid solution N of 0.0040 to 0.0080% by mass can be obtained.
KANEKO SHINJIRO
HIRA TAKAAKI
FURUKIMI OSAMU
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